CHAPTER 3
CELL DIVISION: DNA AND CHROMOSOMES
WHAT IS DNA?
• The genetic material in cells is called DNA –
deoxyribonucleic acid.
• It is a chemical that contains info for an organism’s
growth and function.
• Made of two strands of molecules that twist to look
like a ladder – forming a double helix.
WHERE IS DNA?
• Most of the DNA in a cell is contained in its nucleus.
• It is a mass of loose strands.
• Constantly copied.
• Before division, it is compacted into structures
called chromosomes.
CHROMOSOMES
• A chromosome is made up of two identical
chromatids that are held together in the center by
a centromere.
• Humans have 46 chromosomes, fruit flies have 8,
and corn plants have 20.
CELL DIVISION
• Through cell division, one cell becomes two, two
cells become four, and so on.
• Essential for three function: growth, development,
and repair.
GROWTH
• a large organism does not necessarily have larger
cells than a small organism, it just has more cells.
• Cells do grow, but there are limits to their size.
• Remember, cells need a high surface area to
volume ratio to function, and as it grows the ratio
decreases. When it divides, the surface area of
both new cells increases.
GROWTH
• Also, as a cell grows, its functions must increase.
However, the amount of DNA to demand these
functions remains the same.
DEVELOPMENT
• During development, cells become specialized to
perform specific functions.
• The cells may take on new shapes and structures to
help them complete their task.
REPAIR
• Injuries are repaired through cell division.
• When you have a cut in your skin, skin cells on
either side of the cut divide and replicate. These
new cells heal the wound.
• As cells die and age, they need to be replaced.
Some types of cells need to be replicated at a
faster rate (like skin cells).
CHAPTER 3
THE CELL CYCLE
THE CELL CYCLE
• The normal sequence of development and division
of a cell.
• Two phases: interphase and cell division.
• Cell division is broken into mitosis and cytokinesis
for eukaryotes (cells with nuclei).
INTERPHASE
• The part of the cell cycle during which a cell is not
dividing.
• The cell grows to be about 2x its size.
• Engages in normal cell activities; such as
transporting materials and cellular respiration.
• Towards the end of this phase, DNA is copied
exactly to prepare for cell division.
CELL DIVISION
• Mitosis is the part of the cell cycle when the cell
divides; it is usually the shortest part of the cell cycle.
• Cytokinesis is the division of the parent cell’s
cytoplasm; it happens right after mitosis.
• After these two processes occur, a “parent cell”
becomes two identical “daughter cells” with a
complete set of DNA.
STEPS OF MITOSIS
Step 1: Prophase
• During this phase, the DNA goes from its loose form
into chromosomes.
• The membrane around the nucleus disappears.
STEPS OF MITOSIS
Step 2: Metaphase
• During this phase, the chromosomes line up in the
middle of the cell.
• Hint: Metaphase = Middle
STEPS OF MITOSIS
Step 3: Anaphase
• The chromosome is split into its two chromatids, and
the chromatids move to opposite ends of the cell.
STEPS OF MITOSIS
Step 4: Telophase
• A new nuclear membrane forms around each
group of chromatids, and they return to their loose
threadlike form.
CYTOKINESIS
In animal cells:
• A fiber ring pinches the cell membrane until it is
separated into two individual cells.
In plant cells:
• A cell plate grows between the two nuclei. The cell
plate eventually becomes part of the cell wall for
each of the new cells.
AFTER CELL DIVISION
• There are now two separate cells.
• They are identical to each other, and to their
parent cell.
• They are small cells now, but will go through a
period of growth.
CHAPTER 3
SEXUAL AND ASEXUAL REPRODUCTION
ASEXUAL REPRODUCTION
• One organism produces one or more new
organisms that are identical to itself and that live
independently of it.
• The offspring is genetically identical to its parent.
BINARY FISSION
• A form of asexual reproduction occurring in
prokaryotes.
• The parent splits in two, producing two independent
daughter cells.
• Algae, some yeast, and protozoans (paramecium)
BUDDING
• A form of asexual reproduction occurring in
unicellular and multicellular organisms.
• An organism develops little buds on its body, with
each bud having identical DNA as the parent.
• Some buds stay connected to the parent, but most
detach once they reach a certain size.
• Yeast, hydras, and kalanchoe
REGENERATION
• The process of new tissue growing at injured or lost
limb sites.
• A starfish can be cut in half, and each half will grow
back to form two separate starfish.
• A plant can regrow its cut stems.
SEXUAL REPRODUCTION
• If you grow a plant by cutting it, the new plant is
identical to the parent plant.
• If you grow a plant from a seed, it has genetic
material from two parents.
• Plants growing from seeds and animals growing
from eggs are examples of this.
• Results in greater diversity of living organisms.